Many end uses of esters made from alcohols require the esters to have a low colour, preferably to be water white in colour. Many esters are intensively used as plasticisers for polyvinyl chloride (PVC). Acetate esters are often used as solvents or as fragrance components. Acrylate or methacrylate esters are used as such or as their polymers in the production of coatings, paints, adhesives, in the caulking of building materials, or as binders for leather, paper or textiles. Plasticisers, solvents and fragrance components should have a low colour, and should also have an acceptable odour. Plasticisers should also be resistant to ultra violet light as they are often used in articles exposed to sunlight. The plasticiser should contain only a minimal amount of volatile components or light ends so that it has a low odour level both during its processing and in its final application.
Plasticiser and lubricant esters are typically made by esterification of C6 to C15 alcohols with an acid or anhydride, such as the various isomers of phthalic acid, primarily (ortho-)phthalic anhydride, cyclohexane mono- or dicarboxylic acids or anhydrides, adipic acid, trimellitic acid and its anhydride, or the various isomers of pyromellitic acid or their anhydrides. Acetate, acrylate or methacrylate esters are typically made with C4 to C10 alcohols. The alcohols themselves are in many instances made by employing hydroformylation of olefins; such alcohols are also known as oxo alcohols. Oxo alcohols are primary alcohols and typically have branched alkyl chains. Occasionally they comprise, or are primarily, straight chain primary alcohols. In such case they are made from linear alpha olefins (LAOs), which typically have an even carbon number. The alcohols derived from even carbon numbered LAOs by hydroformylation (which adds a carbon atom to the LAOs) have an odd number of carbon atoms per molecule. Normal or nearly-normal alcohols with even carbon numbers may also be made by hydroformylation.
There are two main synthetic processes for the production of oxo alcohols. One process involves hydroformylation and hydrogenation, possibly preceded by olefin oligomerisation to produce an olefin having a higher carbon number, often referred to as a higher olefin, as the hydroformylation feed. Another main process involves olefin hydroformylation followed by aldol condensation and hydrogenation. The starting materials for all these processes are olefins which may be single olefins or, more commonly, mixtures of olefins. The oligomerisation, hydroformylation, hydrogenation and condensation reactions are all catalysed and tend to involve several different reactions. Accordingly, in many of the steps of both processes, complex reaction mixtures tend to be formed. The final alcohol therefore requires extensive purification to remove unreacted raw materials, undesirable byproducts and catalyst residues. Purification typically involves washing, further hydrogenation or hydrofinishing, and fractional distillation. However, despite rigorous purification, the purified oxo-alcohols invariably contain small amounts of impurities.
Esters are produced by reaction of the appropriate alcohol with an acid or anhydride. In the production of plasticiser ester, the most commonly used anhydrides are phthalic anhydride, trimellitic anhydride, or maleic anhydride. Frequently used acids include adipic acid, trimellitic acid, cyclohexanoic mono- and dibasic acids, benzoic acid, citric acid and the like. Acetates are produced by reaction of the alcohol with acetic acid or its anhydride. Acrylates and methacrylates are typically produced by reaction with acrylic or methacrylic acid.
Alcohol esterification is typically also a catalysed reaction, in many instances an acid catalysed reaction. Examples of acid catalysts include Brönsted acids such as sulfuric acid, methane sulfonic acid and paratoluene sulfonic acid, and Lewis acids such as tin and titanium based organometallic catalysts. Heat is typically provided to the esterification reaction, and it has been found that higher esterification temperatures improve the efficiency of the esterification process.
It is important that the alcohol is stable under the modern esterification conditions, which have become more severe than in the past. One problem with alcohols is that they can develop an undesirable colour under esterification conditions, thus leading to an undesirable colour in the ester formed, which, accordingly, is either not useful, or requires further treatment to reduce the colour back to an acceptable level. We have now found that colour development may occur during esterification using acid catalysts, if small amounts of conjugated unsaturated carbonyl containing compounds are present in the alcohol. By conjugated we mean that the compounds contain ethylenic unsaturation that is conjugated with a carbonyl group in the same molecule. We have also found that conjugated unsaturated aldehydes are greater colour generators than conjugated unsaturated ketones.
Without wishing to be bound by any theory, we believe that the presence of the conjugated unsaturated carbonyl compounds in the alcohol may result from the production of these compounds by virtue of side reactions, particularly dehydration, during one of the steps of the alcohol production process, such as for example an oligomerisation step to produce a higher olefin, the hydroformylation step, the aldol condensation of two aldehydes, or the hydrogenation step. We have also found that these compounds may also result from the presence of impurities in the olefin feeds that are used in the oligomerisation or in the hydroformylation reaction; for example, dienes present in the feed may be hydrated and then dehydrogenated. Surprisingly, some of the conjugated unsaturated carbonyl compounds present in the olefin feeds to hydroformylation appear to be unaffected by the typical distillation and hydrogenation techniques used in commercial alcohol manufacture, and we have found that significant amounts remain in existence through hydroformylation and hydrogenation. They can therefore be present in the alcohol, and in sufficient quantities to have an adverse effect on colour when the alcohol is subjected to acid conditions. We have also found that although both ketones and aldehydes can be formed, they may be formed in oligomerisation coincidentally more ketones than aldehydes.
U.S. Pat. No. 3,232,848 relates to a method of refining synthetically produced alcohols, and to a method of improving the colour quality of synthetic alcohols and their chemical derivatives. U.S. Pat. No. 3,232,848 states that the impurities present in synthetic alcohols are extremely complex in nature and are generally thought to be contaminating amounts of aldehydic and unsaturated compounds, which are introduced into the alcohol by its method of manufacture. The test used for colour in U.S. Pat. No. 3,232,848 is similar to the sulphuric acid test ASTM D 1209-54 which has been superseded a.o. by ASTM E 852-94a, a test which is discussed in more detail, later on in this document. U.S. Pat. No. 3,232,848 suggests that an alcohol of improved colour quality may be provided by a post treatment of the alcohol involving stripping and distillation in the presence of a phosphorous-containing acid.
United Kingdom Patent No. 1336776 is concerned with the production of colourless esters using strong acid catalysts, and states that the presence of trace amounts of carbonyl and ethylenic groups in the alcohols used in the production of the esters results in colouring the esters. The esterification reaction runs for 3 hrs at 135° C. It is stated that the impurities can also develop during esterification, and this document suggests that the problem may be overcome by conducting the esterification reaction in a hydrogenating medium in the presence of a hydrogenation catalyst so that hydrogenation conditions prevail throughout the esterification reaction.
United Kingdom Patent No. 923464 is concerned with the purification of alcohols produced by the Ziegler process, which is an ethylene growth process using aluminium alkyls. It is stated that the presence of contaminants such as aldehydes, esters and aldol-condensation products are capable of imparting colour to alcohols.
United States Patent publication No. 2006/0105465 discloses a colorimetric technique for quantitative analysis of carbonyl functions, i.e. aldehydes and/or ketones in a sample comprising synthetic C4-C15 alcohols obtained by the Oxo process. The technique comprises the reaction of the carbonyl function with 2,4-dinitrophenylhydrazine with an acid to form a hydrazone, which is very fast in the presence of sulphuric acid. Subsequently, the hydrazone may be reacted with potassium hydroxide to form a coloured species, the concentration of which may then be determined by spectrophotometry, preferably using the yellowness colour index according to ASTM E-313. The result is obtainable in a matter of minutes, and therefore, also suitable for application in the field. The technique is shown to analyse for all carbonyl functions, without differentiation. US 2006/0105465 discloses a series of alcohols produced by the Oxo process for which the carbonyl number was determined.
DE 1148221 and related U.S. Pat. No. 3,373,211 are concerned with the presence of carbonyl compounds, saturated or unsaturated, in oxo alcohols and in alcohols prepared by the oxidation and subsequent hydrolysis of aluminum alkyls, and their effect on the colour of the phthalate ester derivative. These documents disclose that such C6 to C16 alcohols, even after extensive fractionation, contain generally from 0.01 to 1.0 wt. percent of carbonyl compounds. A concentration of 0.01 wt % C6 carbonyl compound is equivalent to a carbonyl presence of 1 meq/kg and with a carbonyl number of 0.056 mg KOH/g. A crude stripped decyl alcohol stream, prior to final fractionation and thus containing high molecular weight dimer and trimer product sometimes referred to as oxo bottoms, is disclosed to contain a level of 0.02 wt % carbonyl compounds. The documents do not disclose to what level these carbonyls are concentrated up during the final fractionation, or how much extra carbonyls, saturated or unsaturated, may be formed during such final fractionation, for instance because of oxygen ingress causing oxidative dehydrogenation in the vacuum tower, the entrainment of hydrogenation catalyst fines or of other trace compounds causing side reactions such as the Guerbet alcohol condensation reaction, which mechanism is believed to pass via unsaturated intermediates. The purity requirements for such alcohols have evolved since the 1960's and have nowadays become much more stringent. The same may be said with respect to the disclosure in GB 1252678.
So far, the cause for colour formation in Hot Sulphuric Acid colour tests, such as ASTM E 852-94a or ASTM D5398, has not been clearly identified. The Hot Sulphuric Acid Colour formation may be indicative for the presence of aldehydes, which, in the test, may aldolise and dehydrate to form colour forming bodies. We have also found that the presence of aldehydes in larger amounts may lead to a stronger colour result in the Hot Sulphuric Acid Colour test. Therefore, alcohols intended for esterification should not contain more than a certain level of carbonyl functions, as measured by a known carbonyl test, such as the test described in US 2006/0105465, the disclosure of which is herewith enclosed by reference. Commercial alcohols are therefore typically offered with a maximum carbonyl number as a product specification. Occasionally, a maximum colour reading on the Pt/Co scale according to the Hot Sulphuric Acid Colour test is also specified. It is however not clear which contaminants are relevant for such a specification on the alcohol, nor which levels thereof would be unacceptable. The Hot Sulphuric Acid Colour test therefore is a convenient and indicative tool, but conclusions that may be drawn from its outcome are limited, in particular quantitative conclusions. Therefore, in the production of alcohols, there remains a need for a sensitive and specific measurement method for conjugated unsaturated carbonyl functions, a technique which preferably would be suitable for use in an industrial environment, preferably in the field, such as an at-line or on-line analytical method. Such a method may also be complemented by a convenient carbonyl test for measuring total carbonyls, preferably also as an at-line or even on-line analytical method. Both methods together would then allow for a much closer quality monitoring of alcohol production processes, primarily of the hydrogenation steps in such processes, offering benefits of lower operating temperatures and pressures, longer catalyst lives, lower hydrogen requirements, and less offgasses and spent catalysts to be disposed of They also offer the capability to produce alcohols that are low in unsaturated conjugated carbonyl compounds.
In the present context, an at-line method means an analytical method that is executed by an operator in a field lab, located close to the location where the sample is taken. The sample is typically collected manually by the operator. The analytical method itself may or may not be automated. The advantage is that the result of the analysis is immediately available to the operator. An on-line analytical method is a method that takes its sample on-line, i.e. it is typically fully automated and operates without the intervention of an operator. The analytical procedure of an on-line method is usually also fully automated, so that no human intervention is usually required to generate the result. The result of an on-line analytical method is usually also communicated automatically, such as by electronic means, to more remote observers such as to a panel operator in a centralised control room.
The present invention is concerned with alcohol compositions that contain low levels of conjugated unsaturated carbonyl compounds. The undesirable conjugated unsaturated carbonyl compounds, with which the present invention is concerned, may be present in the alcohol for a variety of reasons. For example, depending upon the nature and origin of the olefin feed to hydroformylation, they may be present in the olefin feed or be developed during processing of the olefin feed. For instance C8 olefins for the production of C9 alcohols can be made by the dimerisation of butenes. Alternatively, butene feeds can be hydroformylated to produce C5 aldehydes which are subject to an aldol reaction and hydrogenated to produce C10 alcohols. Both reactions therefore use butene feeds. Feeds of petroleum origin are often used as the source of butene and these feeds can contain small amounts of butadiene. Furthermore small amounts of butadiene can be produced by dehydrogenation and isomerisation side reactions that can occur during the dimerisation reaction and the hydroformylation reaction, possibly by hydration and further dehydrogenation, and/or combined with hydrogen transfer. The butadiene may therefore be present, and be the precursor of unsaturated carbonyl compounds, some of which may be conjugated. Similarly, small amounts of other dienes may be present in other olefinic feeds and/or be produced during the oligomerisation thereof to produce olefinic feeds for hydroformylation. Conjugated unsaturated carbonyl compounds may also be produced in the hydroformylation reaction, for instance by dehydration of a hydroxy or dihydroxy aldehyde, which may be produced by aldol condensation.